Modified-TEMPO-bed microreactor flow oxidations

It’s nice to read some modified kit approaches to flow methods and this report isn’t an exception. Tyler McQuade reports (Beilstein JOC 2009) a simple approach to putting together a microreactor with fluorpolymer tubing with endcaps and syringe pumps set up to a Y-line to flow through a specially made TEMPO-resin bedding for oxidation of alcohols and aldehydes — to aldehydes and ketones respectively. To stablize the tubing, they were coil-wrapped around metal bars, with the aqueous/organic mixing prior to meeting the oxidant bed. An illustration of the set-up is included below:

Screen Shot 2014-10-20 at 3.49.13 PMScreen Shot 2014-10-20 at 3.49.25 PM

 

Some proper context should probably go along with why this process is particularly useful in concept. Most oxidative processes utilize a metal mediated oxidation and at the end are subject to potential leaching so this offers an example that can be used without an additional purification step or removal of undesired metal content (believe me, I have been around a few precious chiral aldehydes that decomposed with small amounts of metal at the end of the reaction on standing so I appreciate alternate approaches).

The scheme for setting up the reaction:

Screen Shot 2014-10-20 at 3.53.08 PM

Several methods were developed depending on the substrate, solvent and rate of reagent pumping so method A is described to give you an idea of how the reaction was processed. Additional examples are available in the paper.

Screen Shot 2014-10-20 at 4.00.06 PM

Method A: Organic Phase – Alcohol (0.2 M) in CH2Cl2 or EtOAc set to 44 μL min−1 (8.8 μmol alcohol min−1, 1.0 equiv min−1). Aqueous Phase – Aqueous NaOCl (0.25 M), adjusted to pH 9.1 with NaHCO3, mixed with aqueous KBr (0.5 M, 30 µL per mL NaOCl) set to 56 µL min−1 (1.5 equiv NaOCl min−1, 0.10 equiv KBr min−1). The phases combined at a Y-junction and passed through a 60 cm channel packed with AO-TEMPO (300 mg, 0.138 mmol TEMPO) submerged in an ice bath.

An added advantage of the packed column presented itself in the number of additional cycle usage – >100X. Enjoy the creative approach used and happy reading!

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